Download Introduction: Barking Up the Genetic Tree

Chapter 9
Patterns of Inheritance
PowerPoint Lectures for
Biology: Concepts & Connections, Sixth Edition
Campbell, Reece, Taylor, Simon, and Dickey
Copyright © 2009 Pearson Education, Inc.
Introduction: Barking Up the Genetic Tree


Dogs are one of man’s longest genetics experiments
–
Dog breeds are the result of artificial selection
–
Populations of dogs became isolated from each other
–
Humans chose dogs with specific traits for breeding
–
Each breed has physical and behavioral traits due to a unique genetic makeup
Sequencing of the dog’s genome shows evolutionary relationships between breeds
1
Wolf
Ancestral
canine
Chinese Shar-Pei
Akita
Siberian Husky
Basenji
Alaskan Malamute
Afghan hound
Saluki
Rottweiler
Sheepdog
Retriever
9.1 The science of genetics has ancient roots
 Pangenesis (泛生說) was an early explanation for inheritance
– It was proposed by Hippocrates (希波克拉底-被稱為醫學之父)
– Particles called pangenes came from all parts of the organism to be
incorporated into eggs or sperm
– Characteristics acquired during the parents’ lifetime could be
transferred to the offspring
– Aristotle (亞里斯多德) rejected pangenesis and argued that instead of
particles, the potential to produce the traits was inherited
 Blending was another idea, based on plant breeding
– Hereditary material from parents mixes together to form an
intermediate trait, like mixing paint
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2
9.2 Experimental genetics began in an abbey
garden
 Gregor Mendel (孟德爾) discovered principles of genetics in experiments
with the garden pea
– Mendel showed that parents pass heritable factors to offspring
(heritable factors are now called genes)
– Advantages of using pea plants
– Controlled mating
– Self-fertilization or cross-fertilization
– Observable characteristics with two distinct forms
– True-breeding strains
Copyright © 2009 Pearson Education, Inc.
Anatomy of a garden pea flower (with one petal removed to improve visibility)
(花瓣) Petal
(雄蕊) Stamen
(單子房) Carpel
3
Mendel’s technique for crossfertilization of pea plants
White
1
Removed
stamens from
purple flower
Stamens
Carpel
Parents
(P)
2
Purple
3
Transferred
pollen from stamens of white
flower to carpel of purple flower
Pollinated carpel
matured into pod
4
Planted seeds
from pod
Offspring
(F1)
The seven pea characters
studied by Mendel
Flower color
Purple
White
Axial
Terminal
Seed color
Yellow
Green
Seed shape
Round
Wrinkled
Pod shape
Inflated
Constricted
Pod color
Green
Yellow
Tall
Dwarf
Flower position
Stem length
4
9.3 Mendel’s law of segregation describes the
inheritance of a single character
 Example of a monohybrid cross (單性狀雜合體互交;單基因雜種雜交)
– Parental generation: purple flowers  white flowers
– F1 generation: all plants with purple flowers
– F2 generation: 3/4 of plants with purple flowers
1/4 of plants with white flowers
 Mendel needed to explain
– Why one trait seemed to disappear in the F1 generation
– Why that trait reappeared in one quarter of the F2 offspring
Copyright © 2009 Pearson Education, Inc.
P generation
(true-breeding
parents)
Purple flowers
White flowers
5
P generation
(true-breeding
parents)
Purple flowers
White flowers
F1 generation
All plants have
purple flowers
P generation
(true-breeding
parents)
Purple flowers
White flowers
F1 generation
All plants have
purple flowers
Fertilization
among F1 plants
(F1 X F1)
F2 generation
3
–
4
of plants
have purple flowers
1
– of
4
plants
have white flowers
6
9.3 Mendel’s law of segregation describes the
inheritance of a single character

Four Hypotheses:
1. Genes are found in alternative versions called alleles (對偶基因); a
genotype (基因型) is the listing of alleles an individual carries for a
specific gene
2. For each characteristic, an organism inherits two alleles, one from
each parent; the alleles can be the same or different
– A homozygous genotype (同型接合基因型) has identical
alleles
– A heterozygous genotype (異型接合基因型) has two different
alleles
Copyright © 2009 Pearson Education, Inc.
9.3 Mendel’s law of segregation describes the
inheritance of a single character

Four Hypotheses:
3. If the alleles differ, the dominant allele (顯性對偶基因) determines
the organism’s appearance, and the recessive allele (隱性對偶基因)
has no noticeable effect
– The phenotype (表現型) is the appearance or expression of a
trait
– The same phenotype may be determined by more than one
genotype
4. Law of segregation (分離律): Allele pairs separate (segregate) from
each other during the production of gametes so that a sperm or egg
carries only one allele for each gene
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7
Genetic makeup (alleles)
pp
PP
P plants
Gametes
All p
All P
F1 plants
(hybrids)
All Pp
Gametes
1
–
2
1
–
2
P
p
Sperm
P
F2 plants
Phenotypic ratio
3 purple : 1 white
p
P
PP
Pp
p
Pp
pp
Eggs
Genotypic ratio
1 PP : 2 Pp : 1 pp
9.4 Homologous chromosomes bear the alleles
for each character
 For a pair of homologous chromosomes, alleles of a gene reside at the same
locus
– Homozygous individuals have the same allele on both homologues
– Heterozygous individuals have a different allele on each homologue
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8
Matching gene loci on homologous chromosomes
Gene loci
Genotype:
Dominant
allele
P
a
B
P
a
b
Recessive
allele
Bb
PP
aa
Homozygous
Heterozygous
Homozygous
for the
for the
dominant allele recessive allele
9.5 The law of independent assortment is
revealed by tracking two characters at once



Example of a dihybrid cross (雙性狀雜合體互交;雙基因雜種雜交)
–
Parental generation: round yellow seeds  wrinkled green seeds
–
F1 generation: all plants with round yellow seeds
–
F2 generation: 9/16
3/16
3/16
1/16
of plants with round yellow seeds
of plants with round green seeds
of plants with wrinkled yellow seeds
of plants with wrinkled green seeds
Mendel needed to explain
–
Why nonparental combinations were observed
–
Why a 9:3:3:1 ratio was observed among the F2 offspring
Law of independent assortment (獨立分配律):
–
Each pair of alleles segregates independently of the other pairs of alleles
during gamete formation
–
For genotype RrYy, four gamete types are possible: RY, Ry, rY, and ry
9
Two hypotheses for segregation in a dihybrid cross
Hypothesis: Independent assortment
Hypothesis: Dependent assortment
P
generation
rryy
RRYY
ry
Gametes RY
F1
generation
rryy
RRYY
ry
Gametes RY
RrYy
RrYy
Sperm
Sperm
1
–
2
F2
generation
1
–
2
RY
1
–
2
1
–
4
ry
1
–
4
RY
Eggs
1
–
2
RY
1
–
4
ry
rY
1
–
4
Ry
1
–
4
ry
RY
RRYY
RrYY
RRYy
RrYy
RrYY
rrYY
RrYy
rrYy
rY
Eggs
Hypothesized
(not actually seen)
1
–
4
1
–
4
Ry
1
–
4
ry
9
––
16
RRYy
RrYy
RRyy
Rryy
RrYy
rrYy
Rryy
rryy
Actual results
(support hypothesis)
3
––
16
3
––
16
1
––
16
Yellow
round
Green
round
Yellow
wrinkled
Green
wrinkled
Independent assortment of two genes in the Labrador retrievers
Blind
Phenotypes
Genotypes
Black coat, normal vision
B_N_
BbNn
Mating of heterozygotes
(black, normal vision)
Phenotypic ratio
of offspring
Black coat, blind (PRA)
B_nn
9 black coat,
normal vision
3 black coat,
blind (PRA)
Blind
Chocolate coat, normal vision Chocolate coat, blind (PRA)
bbN_
bbnn
BbNn
3 chocolate coat,
normal vision
1 chocolate coat,
blind (PRA)
10
9.6 Geneticists use the testcross to determine
unknown genotypes
 Testcross (測交)
– Mating between an individual of unknown genotype and a
homozygous recessive individual
– Will show whether the unknown genotype includes a recessive allele
– Used by Mendel to confirm true-breeding genotypes
Copyright © 2009 Pearson Education, Inc.
Using a testcross to determine genotype
Testcross:
B_
Genotypes
bb
Two possibilities for the black dog:
BB
B
Gametes
b
Offspring
Bb
or
Bb
All black
b
B
b
Bb
bb
1 black : 1 chocolate
11
9.7 Mendel’s laws reflect the rules of probability
 The probability of a specific event is the number of ways that event can
occur out of the total possible outcomes:
 Rule of multiplication (乘法原則)
– Multiply the probabilities of events that must occur together
 Rule of addition (加法原則)
– Add probabilities of events that can happen in alternate ways
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F1 genotypes
Bb male
Segregation and fertilization as
chance events
Formation of sperm
Bb female
Formation of eggs
1
–
2
1
–
2
1
–
2
B
B
B
b
B
1
–
4
1
–
4
1
–
2
b
B
b
b
B
1
–
4
b
b
1
–
4
F2 genotypes
12
9.8 CONNECTION: Genetic traits in humans can
be tracked through family pedigrees
 A pedigree (譜系)
– Shows the inheritance of a trait in a family through multiple
generations
– Demonstrates dominant or recessive inheritance
– Can also be used to deduce genotypes of family members
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Dominant Traits
Recessive Traits
Freckles
No freckles
Widow’s peak
Straight hairline
Free earlobe
Attached earlobe
Examples of single-gene
inherited traits in humans
13
First generation
(grandparents)
Ff
Second generation
(parents, aunts,
and uncles)
FF
or
Ff
Ff
ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
Third generation
(two sisters)
Female Male
Affected
Unaffected
Pedigree showing inheritance of
attached versus free earlobe in a
hypothetical family
9.9 CONNECTION: Many inherited disorders in
humans are controlled by a single gene
 Inherited human disorders show
– Recessive inheritance
– Two recessive alleles are needed to show disease
– Heterozygous parents are carriers of the disease-causing allele
– Probability of inheritance increases with inbreeding, mating
between close relatives (近親交配)
– Dominant inheritance
– One dominant allele is needed to show disease
– Dominant lethal alleles are usually eliminated from the
population
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14
Parents
Normal
Dd
Offspring produced by parents who are
both carriers for a recessive disorder
D
Offspring
Normal
Dd
X
Sperm
D
d
DD
Normal
Dd
Normal
(carrier)
Dd
Normal
(carrier)
dd
Deaf
Eggs
d
Dr. Michael C. Ain, a specialist
in the repair of bone defects
caused by achondroplasia (軟骨
發育不全) and related disorders
15
9.10 CONNECTION: New technologies can
provide insight into one’s genetic legacy
 Genetic testing of parents
 Fetal testing: biochemical and karyotype analyses
– Amniocentesis (羊膜穿刺)
– Chorionic villus sampling (CVS; 絨毛膜取樣)
 Maternal blood test
 Fetal imaging
– Ultrasound (超音波)
– Fetoscopy (胎兒鏡)
 Newborn screening
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16
Amniocentesis
Chorionic villus sampling (CVS)
Needle inserted
Ultrasound
through abdomen to monitor
extract amniotic fluid
Ultrasound
monitor
Suction tube inserted
through cervix to extract
tissue from chorionic villi
Fetus
Fetus
Placenta
Placenta
Chorionic
villi
Uterus
Cervix
Cervix
Uterus
Amniotic
fluid
Fetal
cells
Centrifugation
Fetal
cells
Several
weeks
Biochemical
tests
Several
hours
Testing a fetus for
genetic disorders
Karyotyping
Ultrasound scanning of a fetus
17
9.11 Incomplete dominance results in
intermediate phenotypes
 Incomplete dominance (不完全顯性): expression of one intermediate
trait
– Neither allele is dominant over the other
– Expression of both alleles is observed as an intermediate phenotype
in the heterozygous individual
Copyright © 2009 Pearson Education, Inc.
P generation
Red
RR
Incomplete dominance
in snapdragon color
White
rr
r
R
Gametes
F1 generation
Pink
Rr
Gametes
1
–
2
R
1
–
2
R
1
–
2
r
Sperm
F2 generation
1
–
2
r
1
–
2
R
RR
rR
1
–
2
r
Rr
rr
Eggs
18
Incomplete dominance in human hypercholesterolemia (高膽固醇血症)
Genotypes:
HH
Homozygous
for ability to make
LDL receptors
Hh
Heterozygous
hh
Homozygous
for inability to make
LDL receptors
Phenotypes:
LDL
LDL
receptor
Cell
Normal
Mild disease
Severe disease
9.12 Many genes have more than two alleles in
the population
 Multiple alleles (多重對偶基因)
– More than two alleles are found in the population
– A diploid individual can carry any two of these alleles
– The ABO blood group has three alleles, leading to four phenotypes:
type A, type B, type AB, and type O blood
 Codominance (共顯性): expression of both alleles
– Neither allele is dominant over the other
– Expression of both alleles is observed as a distinct phenotype in the
heterozygous individual
– Observed for type AB blood
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19
Multiple alleles for the ABO blood groups (codominance)
Blood
Group
(Phenotype) Genotypes
Red Blood Cells
Antibodies
Present in
Blood
Reaction When Blood from Groups Below Is Mixed
with Antibodies from Groups at Left
O
A
B
AB
Anti-A
Anti-B
O
ii
A
IAIA
or
IAi
Carbohydrate A
Anti-B
B
IBIB
or
IBi
Carbohydrate B
Anti-A
AB
IAIB
—
9.13 A single gene may affect many phenotypic
characters
 Pleiotropy (多效性)
– One gene influencing many characteristics
– The gene for sickle cell disease
– Affects the type of hemoglobin produced
– Affects the shape of red blood cells
– Causes anemia (貧血)
– Causes organ damage
– Is related to susceptibility to malaria (瘧疾)
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20
鐮刀型貧血症
Sickle-cell disease,
multiple effects of a single
human gene
Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickle cells
Clumping of cells
and clogging of
small blood vessels
Breakdown of
red blood cells
Physical
weakness
Anemia
Impaired
mental
function
Heart
failure
Paralysis
Pain and
fever
Pneumonia
and other
infections
Accumulation of
sickled cells in spleen
Brain
damage
Damage to
other organs
Rheumatism
Spleen
damage
Kidney
failure
9.14 A single character may be influenced by
many genes
 Polygenic inheritance (多基因遗傳)
– Many genes influence one trait
– Skin color is affected by at least three genes
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21
P generation
A model for polygenic inheritance
of skin color
aabbcc
AABBCC
(very light) (very dark)
F1 generation
AaBbCc
AaBbCc
Sperm
1
–
8
1
–
8
1
–
8
1
–
8
1
–
8
1
–
8
1
–
8
1
–
8
F2 generation
1
–
8
1
–
8
1
–
8
20
––
64
1
–
8
1
–
8
Fraction of population
Eggs
1
–
8
1
–
8
1
–
8
15
––
64
6
––
64
1
––
64
1
––
64
6
––
64
15
––
64
20
––
64
15
––
64
6
––
64
1
––
64
Skin color
9.15 The environment affects many characters
 Phenotypic variations are influenced by the environment
– Skin color is affected by exposure to sunlight
– Susceptibility to diseases, such as cancer, has hereditary and
environmental components
Varying phenotypes due to environmental
factors in genetically identical twins
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22
9.16 Chromosome behavior accounts for
Mendel’s laws
 Mendel’s Laws correlate with chromosome separation in meiosis
– The law of independent assortment depends on alternative orientations
of chromosomes in metaphase I
– The law of segregation depends on separation of homologous
chromosomes in anaphase I
Copyright © 2009 Pearson Education, Inc.
F1 generation
r
The chromosomal basis of
Mendel’s laws
All round yellow seeds
(RrYy)
R
y
Y
R
Y
r
y
Metaphase I
of meiosis
(alternative
arrangements)
r
R
Y
y
The law of independent assortment
23
F1 generation
r
r
R
Y
y
r
R
y
Y
Y
y
Y
The chromosomal basis of
Mendel’s laws
R
All round yellow seeds
(RrYy)
R
Metaphase I
of meiosis
(alternative
arrangements)
The law of independent assortment
r
Anaphase I
of meiosis
y
r
R
Y
y
The law of segregation
r
R
Metaphase II
of meiosis
y
Y
F1 generation
y
y
r
R
Y
y
r
R
y
Y
Y
Y
Y
The chromosomal basis of
Mendel’s laws
R
R
All round yellow seeds
(RrYy)
R
r
r
Metaphase I
of meiosis
(alternative
arrangements)
The law of independent assortment
r
Anaphase I
of meiosis
y
r
R
Y
y
The law of segregation
R
r
Y
y
Y
Y
R
R
Metaphase II
of meiosis
r
R
Y
y
Gametes
y
r
y
Y
Y
r
r
r
1
1
– rY
– ry
4
4
Fertilization among the F1 plants
1
– RY
4
F2 generation
9
:3
:3
y
y
R
R
1
–
4
Ry
:1
24
9.17 Genes on the same chromosome tend to be
inherited together
 Linked Genes (連鎖基因)
– Are located close together on the same chromosome
– Tend to be inherited together
 Example studied by Bateson and Punnett
– Parental generation: plants with purple flowers, long pollen crossed to
plants with red flowers, round pollen
– The F2 generation did not show a 9:3:3:1 ratio
– Most F2 individuals had purple flowers, long pollen or red flowers,
round pollen
Copyright © 2009 Pearson Education, Inc.
Experiment involving linked genes in the sweet pea
Experiment
Purple flower
PpLl
Phenotypes
Purple long
Purple round
Red long
Red round
PpLl
Observed
offspring
284
21
21
55
Long pollen
Prediction
(9:3:3:1)
215
71
71
24
25
Explanation: linked genes
PL
Parental
diploid cell
PpLl
Experiment involving linked
genes in the sweet pea
pl
Meiosis
Most
gametes
pl
PL
Fertilization
Sperm
Most
offspring
PL
pl
PL
PL
PL
pl
pl
pl
PL
pl
PL
Eggs
pl
3 purple long : 1 red round
Not accounted for: purple round and red long
9.18 Crossing over produces new combinations
of alleles
 Linked alleles can be separated by crossing over
– Recombinant chromosomes are formed
– Thomas Hunt Morgan demonstrated this in early experiments
– Geneticists measure genetic distance by recombination frequency
(重組頻率)
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26
Review: Production of recombinant gametes
AB
a b
Tetrad
A B
A b
a B
a b
Crossing over
Gametes
Experiment
Fruit fly experiment demonstrating the
role of crossing over in inheritance:
Parental types are the most frequent offspring, and
the percentage of recombinants can be used to
estimate the genetic distance.
Gray body,
long wings
(wild type)
Black body,
vestigial wings
GgLl
ggll
Female
Male
Gray long
Offspring
Black vestigial Gray vestigial Black long
Drosophila melanogaster (果蠅)
944
965
206
Parental
phenotypes
185
Recombinant
phenotypes
Recombination frequency = 391 recombinants = 0.17 or 17%
2,300 total offspring
Explanation
GgLl
(female)
GL
GL
g l
g l
g l
g l
Gl
gL
ggll
(male)
g l
Sperm
Eggs
GL
g l
Gl
gL
g l
g l
g l
g l
Offspring
27
9.19 Geneticists use crossover data to map genes
 Genetic maps
– Show the order of genes on chromosomes
– Arrange genes into linkage groups representing individual
chromosomes
Q: The distance between gene A and gene B is 30 map units. The
distance between genes B and C is 22 map units. The distance
between A and C is 8 map units. Which gene is in the middle,
between the other two?
A: C is the middle gene. The map is A--------C----------------------B
Copyright © 2009 Pearson Education, Inc.
Mapping genes from crossover data
Chromosome
g
l
c
17%
9%
9.5%
Recombination
frequencies
•
•
Morgan and Sturtevant assumed that the chance of crossing over is
approximately equal at all points along a chromosome.
They hypothesized that the farther apart two genes are on a chromosome, the
higher the probability that a crossover will occur between them.
28
A partial genetic map of a fruit fly chromosome
Mutant phenotypes
Short
aristae
Black
body
(g)
Long aristae Gray
(appendages body
on head)
(G)
Cinnabar Vestigial
eyes
wings
(c)
(l)
Red
eyes
(C)
Normal
wings
(L)
Brown
eyes
Red
eyes
Wild-type phenotypes
9.20 Chromosomes determine sex in many
species
 X-Y system in mammals, fruit flies
– XX = female; XY = male
 X-O system in grasshoppers (蚱蜢) and roaches (蟑螂)
– XX = female; XO = male
 Z-W in system in birds, butterflies, and some fishes
– ZW = female, ZZ = male
 Chromosome number in ants and bees
– Diploid = female; haploid = male
 Temperature-dependent sex determination: crocodilians and many turtles
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29
• The human sex chromosomes:
X chromosome-1336 genes
Y chromosome-307 genes
X
Y
The X-Y system
(male)
44
Parents’
+
diploid
XY
cells
22
+
X
(female)
44
+
XX
22
+
Y
Sperm
22
+
X
44
+
XX
44
+
XY
Offspring
(diploid)
Egg
30
The X-O system
22
+
XX
22
+
X
76
+
ZW
76
+
ZZ
The Z-W system
31
Sex determination by chromosome number
32
16
9.21 Sex-linked genes exhibit a unique pattern of
inheritance
 Sex-linked genes (伴性基因) are located on either of the sex chromosomes
– Reciprocal crosses show different results
– White-eyed female (XrXr)  red-eyed male (XRY)
females (XRXr) and white-eyed males (XrY)
red-eyed
– Homozygous red-eyed female (XRXR)  white-eyed male (XrY)
red-eyed females (XRXr) and red-eyed males (XRY)
– X-linked genes are passed from mother to son and mother to daughter
– X-linked genes are passed from father to daughter
– Y-linked genes are passed from father to son
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32
Fruit fly eye color, a sex-linked characteristic
Homozygous, red-eyed
female X white-eyed male
Female
Male
Xr Y
XR XR
Sperm
Egg
XR
Xr
Y
XR Xr
XR Y
R = red-eye allele
r = white-eye allele
33
Heterozygous, red-eyed
female X red-eyed male
Female
Male
XR Xr
XR Y
Sperm
XR
Y
XR
XR XR
XR Y
Xr
Xr XR
Xr Y
Eggs
Heterozygous, red-eyed
female X white-eyed male
Female
Male
XR Xr
Xr Y
Sperm
Xr
Y
XR
XR XR
XR Y
Xr
Xr Xr
Xr Y
Eggs
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9.22 CONNECTION: Human sex-linked
disorders affect mostly males
 Males express X-linked disorders such as the following when recessive
alleles are present in one copy
– Hemophilia (血友病)
– Colorblindness (色盲)
– Duchenne muscular dystrophy (杜顯氏肌肉萎縮症)
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Hemophilia in the royal family of Russia (half-filled symbols represent
heterozygous carriers)
Queen
Victoria
Albert
Alice
Louis
Alexandra
Czar
Nicholas II
of Russia
Alexis
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9.23 EVOLUTION CONNECTION: The Y
chromosome provides clues about human
male evolution
 Similarities in Y chromosome sequences
– Show a significant percentage of men related to the same male parent
– Demonstrate a connection between people living in distant locations
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Review:
Fertilization
Homologous
Alleles, residing
chromosomes at the same locus
Meiosis
Paired alleles,
alternate forms
of a gene
Gamete
from other
parent
Diploid zygote
(containing
paired alleles)
Haploid gametes
(allele pairs separate)
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Review:
Incomplete
dominance
White
rr
Red
RR
Pink
Rr
Pleiotropy
Single
gene
Multiple characters
(Sickle-cell disease)
Polygenic
inheritance
Multiple
genes
Review:
Genes
located
on
alternative
versions called
(a)
chromosomes
at specific
locations called
(b)
Single characters
(such as skin color)
if both same,
if different,
genotype called genotype called
(c)
heterozygous
expressed
allele called
(d)
unexpressed
allele called
(e)
inheritance when phenotype
In between called
(f)
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You should now be able to
1.
Explain and apply Mendel’s laws of segregation and independent
assortment
2.
Distinguish between terms in the following groups: allele—gene;
dominant—recessive; genotype—phenotype; F1—F2; heterozygous—
homozygous; incomplete dominance—codominance
3.
Explain the meaning of the terms locus, multiple alleles, pedigree,
pleiotropy, polygenic inheritance
4.
Describe the difference in inheritance patterns for linked genes and explain
how recombination can be used to estimate gene distances
5.
Describe how sex is inherited in humans and identify the pattern of
inheritance observed for sex-linked genes
6.
Solve genetics problems involving monohybrid and dihybrid crosses for
autosomal and sex-linked traits, with variations on Mendel’s laws
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